The idea of older guys having (or, rather, not having) sex used to be a popular source of humor. It's probably because, in the days before Viagra, so few men maintained sexual vitality into old age. Even when I was growing up, jokes about an older guy unable to keep up (or get it up) with a young wife were pervasive.
Sex scientists and reproductive medicine researchers have since made this kind of humor obsolete by inventing new drugs and treatments that aid men in retaining virility throughout life.
It's a common meme in vintage humorous postcards to feature heavyset men and women and joke about their weight -- but in the OLD days, the jokes were teasing and gentle. Size wasn't a moral value, and chunky women sometimes came out looking better than their slim peers. In fact, it was widely accepted that some people preferred big, fleshy curves on a woman.
Even when the large woman was the butt (ahem) of the joke, it wasn't attached to the kind of hatred and cruel judgment that today's large woman receives.
Somehow that all changed -- and I can't help wondering if some of the reason it did is because big women are strong women, and strong women are scary as shit.
The best way to disempower and prey on a woman is to make her feel less sexually attractive than other women. And perhaps that is why it's become custom today. Perhaps the more women have been allowed to have social power, the more we subject them to an ugly scrutiny no normal-bodied person could ever possibly survive.